Intercept webbrowser open requests for local html files as well as localhost

[nstrayer]

Addresses #4118

Hooks into the new 'show-html-file' comm path to display html files opened with the webbrowser python module in the viewer pane.

Previously we overrode the webbrowser opening to forward localhost servers, so this just adds a condition to check for html files and do a similar thing.

Here's an example of it working on a bokeh plot that previously used to open in an external browser:

Rough edges:

• Bokeh widget doesn't seem to care about the size of the viewer pane.
• Not sure if how I am checking for the request being an html file is robust enough. I dont think there's an equivalent URI class in python that we can use, right?

QA Notes

[isabelizimm]

Not sure if how I am checking for the request being an html file is robust enough. I dont think there's an equivalent URI class in python that we can use, right?

You could use urlparse from Python's urllib, which returns a named tuple, and check for the file schema and that path ends with .htm/html, but it does not do any sort of validation, if that is what you consider to be robustness.

[seeM]

Confirmed that this works locally, nice!

1. I agree with Isabel's recommendation for urlparse.

2. Could you also please update the test_viewer_webbrowser_does_not_open unit test to include this case? (Maybe the test should be renamed as well.)

3. TODO: Figure out if the file being displayed is a plot or not.

   Any ideas for how to determine this?

[nstrayer]

@seeM ,
1 + 2 (done)
3. Not sure, I think it will depend on what we decide about how we differentiate between the two.

[nstrayer]

@isabelizimm , thanks! Used urlparse in f7a0011

[petetronic]

Just curious, did pytest not work for you?

[nstrayer]

It was trying to use a different python version than which python was giving me. Wasim suggested this change.

[seeM]

My thinking was that python -m pytest should work in all cases where pytest works, and more. So it'll probably save folks some headaches.